Turbofan gas turbine engines (which may be referred to simply as ‘turbofans’) are typically employed to power aircraft. Typically a turbofan gas turbine engine will comprise an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors. The fan is usually driven off an additional lower pressure turbine in the engine core.
The fan comprises an array of radially extending fan blades mounted on a rotor. The fan is surrounded by a fan casing. Generally the fan casing includes a fan track liner positioned so as to circumscribe the fan blades and be proximal thereto.
The precise arrangement of the fan track liner will depend on the engine type and the type of blades used, e.g. metallic or composite blades. However, generally the fan track liner has, in circumferential layers, an abradable layer, an intermediate layer (e.g. an aluminium honeycomb layer), and a septum layer. The septum layer acts as a bonding, separation, and load spreading layer between the abradable layer and the intermediate layer.
Tips of the fan blades are intended to pass as close as possible to the abradable layer when rotating. To achieve this, the abradable layer is dimensioned such that at engine pass-off (i.e. running of the engine before entry into service) one or more of the fan blades cuts a path in the abradable layer so as to achieve a minimal average clearance between the fan blade tips and the fan track liner panel. Typically, the longest fan blades will rub and abrade away the liner by differing amounts over the full 360 degrees circumference, when the engine is operating at its highest power setting. This process advantageously trues the casing and removes any casing asymmetries so as to permit the longest fan blade to run at zero clearance around the circumference of the casing when the engine is running at its highest power setting.
Often the cutting of the path in the abradable layer is done by a single blade tip due to the variation in maximum radial extent of the fan blades. In the present application, maximum radial extent refers to the distance of the tip of the blade from the rotational axis of the fan. The variation in the maximum radial extent of the blades can be attributed to a number of factors including by way of example only, blade length, blade to blade centrifugal growth variation, and in a slotted fan, the disc root slot radial location, and/or the blade retention system. Due to the number of factors that influence the radial extent of the blades further reducing the variation in length during the manufacturing process would be expensive.
It is known for other rotating blades (e.g. blades of the intermediate pressure and high pressure compressor) of a gas turbine engine to grind the blade tips using a blade tip grinding machine that receives the assembled rotary component. The tips of the blades are ground using the machine and then the rotary component is assembled to the remainder of the gas turbine engine. Given the size of the fan compared to the intermediate and high pressure compressor, a much larger and more expensive blade grinding machine would be needed for a fan than for a compressor.